POSILAC® and Reproduction - POSILAC, Bovine Somatotropin (bST) by Monsanto

The reproductive performance of cows receiving supplemental bST has been evaluated in thousands of cows all over the world. The overwhelming conclusion is that the effects of bST are closely associated with energy balance and milk yield. The relationship between milk yield, energy balance, and reproduction has been demonstrated several times in recent years. First service conception rates for all cows have declined from 66% in 1957 to 53% in 1987 (DHIA Data) and milk production has increased more than 6,600 lbs/cow/year during the same time period.¹ The conception rate of heifers has remained consistent at 72% since 1952. Clearly the increase in milk production in multiparous cows is having a negative effect on conception rates. An analysis of 6.4 million lactations also showed that each 200 lbs increase in milk production (305-day lactation) results in a 1-day increase in days open.²

Pooled data from clinical trials in which POSILAC was administered either intramuscularly (IM; Table 1) or subcutaneously (SC; Table 2) is presented. In the IM studies, insemination began at approximately 40 days after calving. As a result, approximately 40% of all cows received at least one insemination prior to the first administration of POSILAC. In the SC studies, all inseminations occurred after the first administration of POSILAC. Therefore, data has not been pooled but is presented separately in Tables 1 and 2.

Table 1
Effect of POSILAC bovine somatotropin (intramuscular injection)
on reproductive performance at 170 days

	Heifers		Cows
	Controls	POSILAC	Controls	POSILAC
Number of animals	56	60	151	156
Services/conception	1.6	2.0	1.8	1.9
*Days open A	89	105	101	103
#Days open B	99	130	115	122
Pregnancy rate, %	90	63	77	70
Multiple births, %	2.9	20.8	1.2	13.6
* Days open A - Average number of days from calving to conception for cows pregnant to term. # Days open B - Average number of days from calving to conception, for cows pregnant full term or to upper limit of breeding period in days postpartum for cows not pregnant full term (eg, if breeding cut-off was 170 days, cows not pregnant by this time were assigned 170 days open).

Table 2
Effect of POSILAC (subcutaneous injection) on reproductive performance at 305 days

	Heifers		Cows
	Controls	POSILAC	Controls	POSILAC
Number of animals	26	27	35	32
Services/conception	2.1	2.4	2.5	2.0
*Days open A	118	119	112	131
#Days open B	135	150	125	162
Pregnancy rate, %	91	83	94	82
Multiple births, %	4.3	4.5	12.1	7.7
* Days open A - Average number of days from calving to conception for cows pregnant to term. # Days open B - Average number of days from calving to conception, for cows pregnant full term or to upper limit of breeding period in days postpartum for cows not pregnant full term (eg, if breeding cut-off was 305 days, cows not pregnant by this time were assigned 305 days open).

From this data set of clinical trials conducted in the U.S., it can be concluded that both heifers and cows experienced a reduction in pregnancy rates and that cows had an increase in days open. POSILAC was also associated with an increased rate of twinning and a small increase in the incidence of cystic ovaries. The length of gestation for cows supplemented with POSILAC was also slightly shorter and calves were lighter at birth; however, by day 30 of life there was no difference in the liveweights of calves from cows supplemented with POSILAC or control cows. Other important indicators of reproductive performance, such as days to first estrus, interestrous interval, services per conception, and the incidence of fetal loss were not affected by supplementation with POSILAC.

When comparisons are made of the reproductive performance of cows of a similar milk production level within the POSILAC bovine somatotropin and control groups, the apparent effects of POSILAC on reproduction are seen to be largely accounted for by the increased milk production.³ Or put more simply, the reproductive performance of cows supplemented with POSILAC is equal to that of unsupplemented cows of an equivalent production level. This is illustrated in Table 3, in which cows within each treatment group were divided into high and low producers. In both the control and treated groups, the higher-producing cows had poorer but similar reproductive performance.

Table 3
Effect of POSILAC and level of milk production on reproductive performance at 305 days

	Heifers		Cows
	Control	POSILAC	Control	POSILAC
Pregnancy rate	88%	85%	89%	85%
Low producers	91%	91%	100%	96%
High producers	85%	79%	78%	74%

Services/conception	2.3	2.3	2.9	2.0
Low producers	2.2	2.9	2.2	1.5
High producers	2.4	1.7	3.6	2.4

* Days open	130	114	130	131
Low producers	113	124	102	114
High producers	147	105	158	148

* Pregnant Cows

Reproductive effects of POSILAC are largely due to increases in milk production. Most studies acknowledge the interrelationships of milk yield, energy balance, and level of dairy herd management on ovarian physiology and heat detection. These interrelationships are not new to the dairy industry. In conclusion, the reproductive performance of the 20,000-lb cow utilizing POSILAC is similar to the 20,000-lb cow without POSILAC.